From what you've posted Pat, it seems to me to be narrowed down to the trailer itself. Either the trailer manufacturer or the dealer has done something different enough to the surface to cause the reaction to the vinyl and which has survived your surface prep, or something is wrong with your surface prep.
To test the theory, try applying a swatch of the same film and a different film and check it after a day or two. If both swatches have bubbles then it is surface contamination. If only the IJ40 has bubbles, then you're dealing with a combination of the IJ40 and the surface as a special situation causing the reaction and you will know that removing and reinstalling with a different film will fix the problem.